1. Field of the Invention
The present invention relates to a tapered roller bearing, and more particularly, to a tapered roller bearing suitably incorporated into the gear device of an automobile transmission.
2. Description of the Related Art
Automobile transmissions are broadly classified into a manual type and an automatic type. Furthermore, they can also be classified according to the driving system of a vehicle into a trans-axle for front wheel drive (FWD), a transmission for rear wheel drive (RWD), and a transfer for four-wheel drive (4WD). These are used to speed-change the drive power delivered from the engine and to transmit it to the drive shaft or the like.
FIG. 7 shows a configuration example of an automobile transmission. This transmission is a synchronous meshing type, in which the left side in the figure is the engine side and the right side is the drive wheel side. A tapered roller bearing 43 is disposed between a main shaft 41 and a main drive gear 42. In this example, the outer ring raceway surface of the tapered roller bearing 43 is directly formed on the inner circumference of the main drive gear 42. The main drive gear 42 is supported using a tapered roller bearing 44 so as to be rotatable with respect to a casing 45. A clutch gear 46 is engaged with and connected to the main drive gear 42, and a synchro-mechanism 47 is disposed adjacent the clutch gear 46.
The synchro-mechanism 47 comprises a sleeve 48 that is moved axially (in the left-right direction in the figure) by the action of a selector (not shown), a synchronizer key 49 installed in the inner circumference of the sleeve 48 so as to be movable in the axial direction, a hub 50 engaged with and connected to the outer circumference of the main shaft 41, a synchronizer ring 51 slidably mounted on the outer circumference (the cone section) of the clutch gear 46, and a urging pin 52 and a spring 53 for elastically pressing the synchronizer key 49 against the inner circumference of the sleeve 48.
In the state shown in the figure, the sleeve 48 and the synchronizer key 49 are held at the neutral position using the urging pin 52. At this time, the main drive gear 42 rotates idle with respect to the main shaft 41. On the other hand, when the sleeve 48 is moved, for example, to the left in the axial direction, from the state shown in the figure by the operation of the selector, the synchronizer key 49 is moved to the left in the axial direction, following the sleeve 48, whereby the synchronizer ring 51 is pressed against the inclined surface of the cone section of the clutch gear 46. This decreases the rotation speed of the clutch gear 46 and increases the rotation speed of the synchro-mechanism 47. Furthermore, at the time when the rotation speeds of the two have become synchronized, the sleeve 48 is further moved to the left in the axial direction, meshing with the clutch gear 46. Hence, the main shaft 41 and the main drive gear 42 are connected to each other via the synchro-mechanism 47. As a result, the main shaft 41 and the main drive gear 42 are rotated synchronously.
In recent years, low-viscosity oil tends to be used for automobile transmissions to meet the needs for automatic transmission (AT), continuously variable transmission (CVT), low fuel consumption, etc. In an environment where low-viscosity oil is used, surface-originated flaking, which causes a very short life, sometimes occurs in the inner ring raceway surface having high surface pressure due to improper lubrication when such adverse conditions as (1) high oil temperature, (2) low amount of oil and (3) loss of pressurization occur simultaneously.
A direct and effective solution to the problem of the short life due to the surface-originated flaking is to reduce the maximum surface pressure. For the purpose of reducing the maximum surface pressure, it is necessary to change the bearing size or to increase the number of the rollers or the bearing if the bearing size is not to be changed. For the purpose or increasing the number of the rollers without decreasing the roller diameter, it is necessary to narrow the distance between the pockets in the cage. However, for this purpose, the pitch circle of the cage must be increased so that the cage is shifted so as to be as close as possible to the outer ring.
As an example in which the cage is shifted so as to make contact with the inner diameter surface of the outer ring, there is a tapered roller bearing shown in FIG. 8 (refer to Japanese Patent Laid-Open No. 2003-28165). In this tapered roller bearing 61, the outer circumferential surface of the small diameter annular section 62a and the outer circumferential surface of the large diameter annular section 62b of the cage 62 are disposed in slide contact with the inner diameter surface of the outer ring 63 so as to guide the cage 62. Furthermore, a recess 64 for suppressing drag torque is formed on the outer diameter surface of the pole section 62c of the cage 62, thereby maintaining the non-contact state between the outer diameter surface of the pole section 62c and the raceway surface 63a of the outer ring 63. The cage 62 has the small diameter annular section 62a, the large diameter annular section 62b, and the multiple pole sections 62c that connect the small diameter annular section 62a to the large diameter annular section 62b in the axial direction and are formed with the recess 64 on the outer diameter surface thereof. Furthermore, multiple pockets, in each so which a tapered roller 65 is rollably accommodated, are provided so that each pocket is disposed between two pole sections 62c. The small diameter annular section 62a is provided with a flange section 62d integrally extending to the inner diameter side. The tapered roller bearing shown in FIG. 8 is an example intended to improve the strength of the cage 62, wherein the cage 62 is shifted so as to make contact with the inner diameter surface of the outer ring 63 in order to increase the circumferential width of the pole section 62c of the cage 62.
In the tapered roller bearing 61 described in Japanese Patent Laid-Open No. 2003-28165, the cage 62 is shifted to the outer diameter side so as to make contact with the inner diameter surface of the outer ring 63 in order to increase the circumferential width of the pole section 62c of the cage 62. Furthermore, because the recess 64 is provided in the pole section 62c of the cage 62, the plate thickness of the pole section 62c becomes inevitably thin, and the rigidity of the cage 62 is reduced. Hence, the cage 62 may be deformed due to stress during the assembly of the bearing 61 or may also be deformed during the rotation of the bearing 61.
On the other hand, a conventional typical tapered roller bearing with a cage, other than the tapered roller bearing described in Japanese Patent Laid-Open No. 2003-28165, is designed so that the roller coefficient γ (roller filling factor) defined by the following formula is usually 0.94 or less in order to securely obtain the pole width of the cage 72 and obtain appropriate strength of the pole of the cage 72 and smooth rotation while avoiding contact between the outer ring 71 and the cage 72 as shown in FIG. 9.Roller coefficient γ=(Z·DA)/(π·PCD)where Z is the number of the rollers, DA is the average diameter of the rollers, and PCD is the pitch circle diameter of the cage.
In addition, in FIG. 9, numeral 73 denotes the tapered roller, numeral 74 denotes the surface of the pole, numeral 75 denotes the inner ring, and e denotes a window angle.